Method and apparatus for coding video, and method and apparatus for decoding video accompanied by inter prediction using collocated image

ABSTRACT

Provided is an inter prediction method including determining a collocated block of a current block of a current image from among blocks of an image that is restored prior to the current image; preferentially checking whether a first reference list from among reference lists of the collocated block is referred to and selectively checking whether a second reference list is referred to according to whether the first reference list is referred to; based on a result of the checking, determining a single collocated reference list from among the first reference list and the second reference list; determining a reference block of the current block by using motion information of the collocated reference list; and performing inter prediction on the current block by using the determined reference block.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.15/057,361, filed on Mar. 1, 2016 which is a is a continuation of U.S.application Ser. No. 14/671,466, filed on Mar. 27, 2015, in the U.S.Patent and Trademark Office, issued as U.S. Pat. No. 9,313,517 on Apr.12, 2016, which is a continuation of U.S. application Ser. No.14/130,545, filed on Jan. 16, 2014, in the U.S. Patent and TrademarkOffice, issued as U.S. Pat. No. 9,253,488 on Feb. 2, 2016, which is aNational Stage application under 35 U.S.C. §371 of PCT/KR2012/005247,filed on Jul. 2, 2012, which claims the benefit of U.S. ProvisionalApplication No. 61/504,177 filed on Jul. 2, 2011, and U.S. ProvisionalApplication No. 61/548,415, filed on Oct. 18, 2011, the disclosures ofwhich are incorporated herein by reference in their entireties.

BACKGROUND

1. Field

Exemplary embodiments relate to a method and apparatus for encoding avideo via inter prediction and motion compensation and a method andapparatus for decoding a video via inter prediction and motioncompensation.

2. Related Art

As hardware for reproducing and storing high resolution or high qualityvideo content is being developed and supplied, a need for a video codecfor effectively encoding or decoding the high resolution or high qualityvideo content is increasing. According to a related video codec, a videois encoded according to a limited encoding method based on a macroblockhaving a predetermined size.

Image data of a spatial region is transformed into coefficients of afrequency region via frequency transformation. According to a videocodec, an image is split into blocks having a predetermined size,discrete cosine transformation (DCT) is performed for each respectiveblock, and frequency coefficients are encoded in block units, for rapidcalculation of frequency transformation. Compared with image data of aspatial region, coefficients of a frequency region are easilycompressed. In particular, since an image pixel value of a spatialregion is expressed according to a prediction error via inter predictionor intra prediction of a video codec, when frequency transformation isperformed on the prediction error, a large amount of data may betransformed to 0. According to a video codec, an amount of data may bereduced by replacing data that is consecutively and repeatedly generatedwith small-sized data.

SUMMARY

Aspects of one or more exemplary embodiments provide an inter predictionmethod and apparatus for determining a reference image by using acollocated picture, a video encoding method and a video decoding methodvia inter prediction, and a video decoding method and a video decodingapparatus via inter prediction.

According to aspects of one or more exemplary embodiments, there isprovided an inter prediction method including determining a collocatedblock of a current block of a current image from among blocks of animage that is restored prior to the current image; preferentiallychecking whether a first reference list from among reference lists ofthe collocated block is referred to and selectively checking whether asecond reference list is referred to according to whether the firstreference list is referred to; based on a result of the checking,determining a single collocated reference list from among the firstreference list and the second reference list; determining a referenceblock of the current block by using motion information of the collocatedreference list; and performing inter prediction on the current block byusing the determined reference block.

Also, according to aspects of one or more exemplary embodiments, theinter prediction apparatus may preferentially check the first referencelist including reference images positioned in an opposite direction to adirection from the current block to the collocated block in thecollocated picture without checking all of a plurality of referencesimages included in the reference list of the collocated block in orderto determine the reference image of the current block. The interprediction apparatus may selectively check the remaining referencelists. Thus, an unnecessary process is skipped in a process fordetermining the reference image of the current block by using thecollocated block, thereby increasing the efficiency of a process ofdetermining the reference image for inter prediction.

According to an aspect of an exemplary embodiment, there is provided aninter prediction method including determining a collocated block of acurrent block of a current image from among blocks of an image that isrestored prior to the current image; preferentially checking whether afirst reference list from among reference lists of the collocated blockis referred to and selectively checking whether a second reference listis referred to according to whether the first reference list is referredto; based on a result of the checking, determining a single collocatedreference list from among the first reference list and the secondreference list; determining a reference block of the current block byusing motion information of the collocated reference list; andperforming inter prediction on the current block by using the determinedreference block.

The first reference list may include images that are positioned oppositeto a direction from the current image to the collocated block.

The selective checking of the second reference list may include when thefirst reference list is referred to for inter prediction, skipping anoperation for checking whether the second reference list is referred to.

The determining of the collocated reference list may include, when apicture order count (POC) number of an image of the collocated block isalways smaller than that of the current image, determining a referencelist of the current block as the single collocated reference list.

The selective checking of the second reference list may include checkingthe first reference list or the second reference list according towhether motion information of the first reference list or the secondreference list exists.

According to another aspect of an exemplary embodiment, there isprovided an inter prediction apparatus including a collocated referencelist checking unit for determining a collocated block of a current blockof a current image from among blocks of an image that is restored priorto the current image, and preferentially checking whether a firstreference list from among reference lists of the collocated block isreferred to and selectively checking whether a second reference list isreferred to according to whether the first reference list is referredto; a reference block determiner for, based on a result of the checking,determining a single collocated reference list from among the firstreference list and the second reference list, and determining areference block of the current block by using motion information of thecollocated reference list; and an inter prediction unit for performinginter prediction on the current block by using the determined referenceblock.

According to another aspect of an exemplary embodiment, there isprovided a video decoding apparatus including a parser for performingentropy decoding on a bit string obtained by parsing a received bitstream to restore samples; an inverse transformer for performing inversequantization and inverse transformation on a quantized transformationcoefficient from among the restored samples to restore samples; an intrapredictor for performing intra prediction on blocks in an intraprediction mode from among the samples restored by the inversetransformer; and a motion compensator for preferentially checkingwhether a first reference list from among reference lists of acollocated block of the current block, selectively checking whether asecond reference list is referred to according to whether the firstreference list is referred to, determining a single collocated referencelist from among the first reference list and the second reference listbased on a result of the checking, and performing inter prediction onthe current block by using a reference block of the current block basedon motion information of the collocated reference list, for performinginter prediction on a current block in an inter mode from among thesamples restored by the inverse transformer; and a restorer forrestoring an image by using blocks that are restored via the interprediction or the intra prediction.

According to another aspect of an exemplary embodiment, there isprovided a video encoding apparatus including an intra predictor forperforming intra prediction on blocks in an intra prediction mode fromamong blocks of a video; an inter predictor for preferentially checkingwhether a first reference list from among reference lists of acollocated block of the current block is referred to, selectivelychecking whether a second reference list is referred to according towhether the first reference list is referred to, determining a singlecollocated reference list from among the first reference list and thesecond reference list based on a result of the checking, and performinginter prediction on the current block by using a reference block of thecurrent block based on motion information of the collocated referencelist, for inter prediction of a current block in an inter mode; atransformation quantizer for performing transformation and quantizationon a result of the intra prediction or the inter prediction; and anoutput unit for outputting a bitstream generated by performing entropyencoding on samples including a quantized transformation coefficientgenerated as a result of the transformation and the quantization.

According to another aspect of an exemplary embodiment, there isprovided a computer readable recording medium having recorded thereon aprogram for executing the inter prediction method.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of an inter prediction apparatus according toan exemplary embodiment;

FIG. 2 shows a method of determining a reference image by using acollocated block;

FIG. 3 shows a method of determining a reference image by using acollocated block, according to an exemplary embodiment;

FIG. 4 is a flowchart of an inter prediction method according to anexemplary embodiment;

FIG. 5 is a flowchart of a video encoding method via inter predictionaccording to an exemplary embodiment;

FIG. 6 is a flowchart of a video decoding method via inter predictionaccording to an exemplary embodiment;

FIG. 7 is a block diagram of a video encoding apparatus based on acoding unit according to a tree structure, according to an exemplaryembodiment;

FIG. 8 is a block diagram of a video decoding apparatus based on acoding unit according to a tree structure, according to an exemplaryembodiment;

FIG. 9 is a diagram for describing a concept of coding units accordingto an exemplary embodiment;

FIG. 10 is a block diagram of an image encoder based on coding unitsaccording to an exemplary embodiment;

FIG. 11 is a block diagram of an image decoder based on coding unitsaccording to an exemplary embodiment;

FIG. 12 is a diagram illustrating deeper coding units according todepths, and partitions according to an exemplary embodiment;

FIG. 13 is a diagram for describing a relationship between a coding unitand transformation units, according to an exemplary embodiment;

FIG. 14 is a diagram for describing encoding information of coding unitscorresponding to a coded depth, according to an exemplary embodiment;

FIG. 15 is a diagram of deeper coding units according to depths,according to an exemplary embodiment;

FIGS. 16 through 18 are diagrams for describing a relationship betweencoding units, prediction units, and transformation units, according toan exemplary embodiment; and

FIG. 19 is a diagram for describing a relationship between a codingunit, a prediction unit or a partition, and a transformation unit,according to encoding mode information of Table 1.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

Hereinafter, an inter prediction method and apparatus using a referencelist of a collocated block will be described with reference to FIGS. 1through 5. A video encoding method and a video decoding apparatus viainter prediction will be described with reference to FIGS. 5 and 6. Inaddition, a video encoding method and a video decoding apparatus viainter prediction based on a coding unit having a tree structure will bedescribed with reference to FIGS. 7 through 19. Hereinafter, the term‘image’ may refer to a still image or a moving picture, that is, a videoitself.

First, with reference to FIGS. 1 through 4, an inter prediction methodand an inter prediction apparatus using a reference list of a collocatedblock according to an exemplary embodiment will be described. Inaddition, with reference to FIGS. 5 and 6, a video encoding method and avideo decoding method via inter prediction according to an exemplaryembodiment will be described.

FIG. 1 is a block diagram of an inter prediction apparatus 10 accordingto an exemplary embodiment.

The inter prediction apparatus 10 includes a reference list checkingunit 12 (e.g., a reference list checker), a reference block determiningunit 14 (e.g., a reference block determiner), and an inter predictionunit (e.g., an inter predictor) 16.

The inter prediction apparatus 10 encodes each video image for eachrespective block. A block may have a square shape, a rectangular shape,or any geometric shape and is not limited to a data unit having apredetermined size. According to an exemplary embodiment, a block may bea maximum coding unit, a coding unit, a prediction unit, atransformation unit, or the like from among coding units according to atree structure. Video encoding and decoding methods based on codingunits according to a tree structure will be described below withreference to FIGS. 7 through 19.

The reference list checking unit 12 may determine a collocated block ofa current block of a current image from among blocks of an image that isrestored prior to the current image. The collocated block of the currentblock of the current image may be determined from among the blocks ofthe image that is restored prior to the current image, and then, acollocated block positioned at a block location in a collocated picture,corresponding to a block location of the current block in the currentimage, may be determined.

The reference list checking unit 12 may determine a reference list ofthe current block by using a reference list of the collocated block.

The reference list checking unit 12 may check whether a first referencelist from among reference lists of the collocated block ispreferentially referred to. The first reference list according to thepresent embodiment may include images that are positioned in an oppositedirection to a direction from the current image to the collocated blockin the collocated block.

The reference list checking unit 12 may selectively check whether asecond reference list is referred to, according to whether the firstreference list is referred to. When the first reference list is referredto, it does not have to be checked whether the second reference list isreferred to.

When the first reference list is referred to for inter prediction of thecollocated block, the reference list checking unit 12 may skip a processof checking whether the second reference list is referred to.

The reference list checking unit 12 may check whether the firstreference list or the second reference list is referred to, according towhether motion information of the first reference list or the secondreference list exists.

The reference block determining unit 14 may determine a reference blockof the current block according to a result of the checking whether thefirst reference list or the second reference list is referred to.

The reference block determining unit 14 may determine a singlecollocated reference list from among the first reference list and thesecond reference list. When the reference block determining unit 14checks that the first reference list is capable of being referred to,the reference block determining unit 14 determines that the firstreference list is the collocated reference list. When the referenceblock determining unit 14 checks that the second reference list iscapable of being referred to, the reference block determining unit 14determines that the second reference list is the collocated referencelist.

The reference block determining unit 14 may determine the referenceblock of the current block by using motion information of the collocatedreference list. A collocated reference image may be determined accordingto the collocated reference list. A reference image of the current imagemay be determined according to a direction and distance from thecollocated picture to the collocated reference image. In addition,motion information of the current block may be determined by modifyingmotion information of the collocated reference list in proportion to thedirection and distance from the collocated picture to the collocatedreference image, and the reference block may be determined in thereference image of the current image according to the modified motioninformation of the collocated reference list.

However, when a picture order count (POC) number of an image of thecollocated block is always smaller than that of the current image, thereference block determining unit 14 may replace the collocated referencelist with the reference list of the current block. Thus, the referenceimage of the current block may be determined according to the referencelist of the current block.

The reference block determining unit 14 may determine the referenceimage of the current block according to the reference list of thecurrent block in a low-delay condition for preventing video encodingfrom being delayed. For example, when a list 0 and a list 1 of thereference list of the current block include the same reference images,that is, in a generalized P and B (GPB) mode, the reference image may bedetermined according to the reference list of the current block. When acurrent condition to decode an image in satisfies the low-delaycondition, the reference block determining unit 14 may determine thereference image of the current block according to the reference list ofthe current block.

The inter prediction unit 16 may perform inter prediction on the currentblock by using the reference block determined by the reference blockdetermining unit 14.

The inter prediction apparatus 10 may include a central processor (notshown) for generally controlling the reference list checking unit 12,the reference block determining unit 14, and the inter prediction unit16. Alternatively, the reference list checking unit 12, the referenceblock determining unit 14, and the inter prediction unit 16 may becontrolled by respective processors (not shown) and the processors maycooperatively interact with each other so as to control an overalloperation of the inter prediction apparatus 10. Alternatively, thereference list checking unit 12, the reference block determining unit14, and the inter prediction unit 16 may be controlled according tocontrol of an external processor (not shown) of the inter predictionapparatus 10.

The inter prediction apparatus 10 may include at least one data storageunit (not shown) for storing data that is input to and output from thereference list checking unit 12, the reference block determining unit14, and the inter prediction unit 16. The inter prediction apparatus 10may include a controller (not shown) for controlling input/output ofdata of a data storage unit (not shown).

The inter prediction apparatus 10 may preferentially check the firstreference list including reference images positioned in an oppositedirection to a direction from the current block to the collocated blockin the collocated picture. The inter prediction apparatus 10 mayselectively check the remaining reference lists, without checking all ofa plurality of references images included in the reference list of thecollocated block, in order to determine the reference image of thecurrent block.

When the inter prediction apparatus 10 checks that the first referencelist of the collocated block is used for inter prediction of thecollocated picture, since the inter prediction apparatus 10 maydetermine the reference image of the current block based on the firstreference list of the collocated block, a process for rechecking whetherthe remaining references of the collocated block are referred to may beskipped. Thus, an unnecessary process is skipped in a process fordetermining the reference image of the current block by using thecollocated block, thereby increasing the efficiency of a process ofdetermining the reference image for inter prediction.

FIG. 2 shows a method of determining a reference image by using acollocated block.

A reference image of a current block 25 of a current image 20 may bedetermined with reference to a reference list of a collocated block 27of the current block 25.

Indexes of reference lists may be expressed by List 0 28 and List 1 29.According to a POC order of images 22, 20, 21, and 23, a reference listincluding reference images ahead of the current image 20 may beexpressed by List 0 L0 and reference images including reference imagesbehind the current image 20 may be expressed by List 1 L1.

A value ‘colDir’ of a collocated picture 21 of the current block 25indicates a direction toward the collocated picture 21. Since thecollocated picture 21 is included in a list 1 26 of the current image20, the ‘colDir’ may be 1. As another example, a‘collocated_from_10_flag’ value may be used as a parameter for searchingfor the collocated picture 21. The ‘collocated_from_10_flag’ value mayindicate that the collocated picture 21 is an image of the list 0 of thecurrent image 20. Thus, the value ‘collocated_from_10_flag’ of thecurrent image 20 may be determined as 0.

The collocated block 27 may be positioned at a block location in thecollocated picture 21, corresponding to a block location of the currentblock 25 in the current image 20. In a related method, a reference imageof the current block 25 may be determined by checking whether both alist 0 28 and a list 1 29 of a reference list of the collocated block 27are referred to.

Typically, the reference image of the current block 25 may be determinedfrom the collocated block 27 in a reference direction across the currentimage 20. Since the reference direction across the current image 20 fromthe collocated block 27 is a direction toward the list 0 28, thereference image of the current block 25 is likely to be positioned inthe direction toward the list 0 28. Thus, in the related method, even ifa process of checking whether the list 1 29 is referred to is likely tobe unnecessary, whether both the list 0 28 and the list 1 29 of thereference list of the collocated block 27 are referred to needs to bechecked.

FIG. 3 shows a method of determining a reference image by using acollocated block, according to an exemplary embodiment.

Generally, a reference image of a current block 35 may be determinedfrom a collocated block 37 in a reference direction across a currentimage 30. That is, if a collocated picture 31 is included in a list 1 36of the current block 35, the reference image of the current block 35 islikely to be determined from the collocated block 37 in a referencedirection toward a list 0 38 across the current image 30.

If another collocated picture is positioned in the reference directiontoward the list 0 38, the reference image of the current block 35 islikely to be determined from the collocated picture in a referencedirection toward the list 1 36 across the current image 30.

Thus, according to the present embodiment, in order to determine thereference image of the current block 35, the inter prediction apparatus10 may preferentially check whether a single reference list from amongreference lists, that is, the lists 0 and 1 38 and 39 of a collocatedblock 37 is referred to. Whether a corresponding reference list isreferred to may be determined according to whether the collocated block37 has motion information about the corresponding reference list as aresult of whether the corresponding reference list has been previouslyreferred to during restoring the collocated block 37.

If the reference list that is preferentially checked has not been usedfor inter prediction of the collocated block 37, the inter predictionapparatus 10 may check whether the remaining reference list of thecollocated block 37 is referred to.

As described above, a reference list may be determined from thecollocated block 37 in the reference direction across the current image30. Thus, if the collocated picture 31 is included in the list 1 36 ofthe current block 35, the inter prediction apparatus 10 may checkwhether the list 0 38 is referred to from the collocated block 37 alonga direction across the current image 30. When it is determined that thelist 0 38 is referred to, it does not have to be checked whether a list1 39 is referred to. However, if images of the list 0 38 of thecollocated block 36 are not referred to for inter prediction, the interprediction apparatus 10 may simply check whether the list 1 39 of thecollocated block 36 is referred to.

Similarly, if a collocated picture of a current block is included in alist 0 of the current block, the inter prediction apparatus 10 maypreferentially check whether a list 1 of a collocated block is referredto.

Thus, the inter prediction apparatus 10 may determine a reference listthat is subject to an operation of preferentially checking whether thereference list is referred to, from among reference lists of acollocated block, based on a reference direction from a current block toa collocated picture.

That is, the inter prediction apparatus 10 determines a direction towarda reference list that is subject to an operation of preferentiallychecking whether the reference list is referred to, from among referencelists of a collocated block, as an opposite direction to the referencedirection from the current block to the collocated picture. Thus, if thecollocated picture is an image of a list 0 of the current image, whethera list 1 of the collocated block is referred to may be preferentiallychecked. If the collocated picture is an image of the list 1 of thecurrent image, whether the list 0 of the collocated block is referred tomay be preferentially checked.

For example, a reference list that is subject to an operation ofpreferentially checking whether the reference list is referred to fromamong reference lists of the collocated block may be determined oppositeto a reference direction from the current block to the collocatedpicture. Thus, when the reference direction from the current block tothe collocated picture is expressed by ‘colDir’, the inter predictionapparatus 10 may determine a reference list that is subject to anoperation of preferentially checking whether the reference list isreferred to along ‘1-colDir’, from among reference lists of thecollocated block.

As another example, when a collocated picture is an image of a list 0 ofa current image, a value ‘collocated_from_10_flag’ of a current blockis 1. When the collocated picture is an image of a list 1 of the currentimage, the value ‘collocated_from_10_flag’ is 0. Thus, the interprediction apparatus 10 may determine a direction toward a referencelist that is subject to an operation of preferentially checking whetherthe reference list is referred to from among reference lists of thecollocated block according to the value ‘collocated_from_10_flag’ of thecurrent block.

Thus, the inter prediction apparatus 10 may determine the referenceblock of the current block by using motion information of a collocatedreference list that is selected based on whether the first referencelist is referred to.

However, in a low-delay condition, the inter prediction apparatus 10 maydetermine the reference image of the current block based on thereference list of the current block, instead of the reference list ofthe collocated block. For example, when a POC number of an image of thecollocated block is always smaller than that of the current image, orwhen a predetermined condition including a GPB prediction mode, in whichlists 0 and 1 of reference lists of the current block include the samereference images, is satisfied, an image is decoded in the low-delaycondition. In the low-delay condition, the inter prediction apparatus 10may replace the collocated reference list with the reference list of thecurrent block and then may determine the reference block of the currentblock by using motion information of the collocated reference list.

FIG. 4 is a flowchart of an inter prediction method according to anexemplary embodiment.

In operation 41, a collocated block of a current block of a currentimage is determined from among blocks of an image that is restored priorto the current image.

In operation 42, whether a first reference list is preferentiallyreferred to from among reference lists of the collocated block ischecked, and whether a second reference list is referred to is checkedaccording to whether the first reference list is referred to.

According to the present embodiment, the first reference list mayinclude images that are positioned opposite to a direction from thecurrent image to the collocated block. When the first reference list isreferred to for inter prediction of the collocated block, a process ofchecking whether the second reference list is referred to may beskipped.

In operation 43, based on a result of the checking of operation 42, asingle collocated reference list is determined from the first referencelist and the second reference list. When a video is decoded in thelow-delay condition, the reference list of the current block isdetermined as a collocated reference list and a reference image may bedetermined according to the reference list of the current block.

In operation 44, a reference block of the current block is determined byusing motion information of the collocated reference list. In operation45, inter prediction is performed on the current block by using thereference block determined in operation 44.

Thus, in the method of determining a reference image for interprediction according to the present embodiment, if it is checked thatthe first reference list of the collocated block is used for interprediction of the collocated picture, an unnecessary process forrechecking whether the remaining reference lists of the collocated blockare referred to may be skipped, thereby increasing the efficiency ofinter prediction.

FIG. 5 is a flowchart of a video encoding method via inter predictionaccording to an exemplary embodiment.

In operation 51, intra prediction is performed on blocks in an intraprediction mode from among blocks of a video.

In operation 52, it is checked whether a first reference list from amongreference lists of a collocated block of a current block ispreferentially referred to, for inter prediction of the current block inan inter mode. The first reference list may include images that arepositioned in an opposite direction to a direction from the currentimage to the collocated block.

When the first reference list is capable of being referred to, it doesnot have to be checked whether a second reference list is referred to.When the first reference list is not referred to, whether the secondreference list is referred to may be checked. Based on a result of thechecking, a single collocated reference list may be determined fromamong the first reference list and the second reference list and areference block of the current block may be determined based on motioninformation of the collocated reference list. Inter prediction may beperformed on the current block by using the reference block of thecurrent block to generate a residual value.

In operation 53, transformation and quantization are performed on theresult of intra prediction or inter prediction to generate a quantizedtransformation coefficient. In operation 55, a bitstream generated byperforming entropy encoding on samples including the quantizedtransformation coefficient of operation 53 is output. A parameter‘colDir’ indicating a direction toward the collocated picture of thecurrent block or a parameter ‘collocated_from_10_flag’ indicatingwhether the current image of the collocated picture is an image of list0 may be transmitted.

In addition, during the inter prediction of operation 52, when an imageis restored in a low-delay condition, a reference image may bedetermined according to the reference list of the current blockregardless of the collocated reference list.

A video encoding apparatus performing the video encoding method of FIG.5 may include the inter prediction apparatus 10 according to anexemplary embodiment. The video encoding apparatus including the interprediction apparatus 10 may perform intra prediction, inter prediction,transformation, and quantization for each image block to generatesamples and may perform entropy encoding on the samples to generate abitstream. In the video encoding apparatus including the interprediction apparatus 10, the inter prediction apparatus 10 may interactwith a video encoding processor or an external video encoding processor,which is mounted in the video encoding apparatus to perform a videoencoding operation including transformation, in order to output a videoencoding result. According to an exemplary embodiment, in an internalvideo encoding processor of the video encoding apparatus, since a videoencoding apparatus, a central processing apparatus, or a graphicprocessing apparatus may include a video encoding module as well as aseparate processor, a basic video encoding operation may be performed.

FIG. 6 is a flowchart of a video decoding method via inter predictionaccording to an exemplary embodiment.

In operation 61, entropy decoding is performed on a bit string obtainedby parsing a received bit stream to restore samples. In operation 62,inverse quantization and inverse transformation are performed on aquantized transformation coefficient from among the samples to restorethe samples. In operation 63, intra prediction is performed on samplesin an intra mode. In operation 64, motion compensation is performed onsamples in an inter mode. In operation 65, an image is restored by usingblocks that are restored via the intra prediction of operation 63 or themotion compensation of operation 64.

In operation 64, a collocated block of a current block is determinedfrom among samples, for inter prediction of a current block in an intermode. A parameter ‘colDir’ indicating a direction toward the collocatedpicture of the current block or a parameter ‘collocated_from_10_flag’indicating whether the current image of the collocated picture is animage of list 0 may be parsed from a bitstream and restored. Thecollocated block of the current block may be determined based on theparameter ‘colDir’ or the parameter ‘collocated_from_10_flag’.

Whether a first reference list from among reference lists of thecollocated block is referred to is preferentially checked. The firstreference list may include images that are positioned in an oppositedirection to a direction from the current image to the collocated block.

When the first reference list is capable of being referred to, it doesnot have to be checked whether a second reference list is referred to.When the first reference list is not referred to, whether the secondreference list is referred to may be checked. Based on a result of thechecking, a single collocated reference list may be determined fromamong the first reference list and the second reference list and areference block of the current block may be determined based on motioninformation of the collocated reference list. Motion compensation of thecurrent block may be performed on the current block by using thereference block of the current block to generate a block pixel samplevalue.

In addition, during the motion compensation of operation 63, when animage is restored in a low-delay condition, a reference image may bedetermined according to a reference list of the current block,regardless of the collocated reference list.

A video decoding apparatus performing the video decoding method of FIG.6 may include the inter prediction apparatus 10 according to anexemplary embodiment. The video decoding apparatus including the interprediction apparatus 10 may parse samples obtained by encoding abitstream and may perform inverse quantization, inverse transformation,intra prediction, and motion compensation for each image block torestore samples. In the video decoding apparatus, the inter predictionapparatus 10 may interact with a video encoding processor or an externalvideo encoding processor, which is mounted in the video decodingapparatus to perform a video decoding operation including inversetransformation or prediction/compensation, in order to output a videodecoding result. According to an exemplary embodiment, in an internalvideo decoding processor or the video decoding apparatus, since a videodecoding apparatus, a central processing apparatus, or a graphicprocessing apparatus may include a video encoding module as well as aseparate processor, a basic video decoding operation may be performed.

In the inter prediction apparatus 10, blocks obtained by dividing videodata are divided into coding units having a tree structure andprediction units are used for inter prediction of the coding units, asdescribed above. Hereinafter, with reference to FIGS. 7 through 19, amethod and apparatus for encoding a video and a method and apparatus fordecoding a video based on a coding unit having a tree structure and acoding unit will be described.

FIG. 7 is a block diagram of a video encoding apparatus 100 based on acoding unit according to a tree structure, according to an exemplaryembodiment.

The video encoding apparatus 100 via video prediction based on a codingunit according to a tree structure includes a maximum coding unitsplitter 110, a coding unit determiner 120, and an output unit 130(e.g., an output). Hereinafter, for convenience of description, thevideo encoding apparatus 100 via video prediction based on a coding unitaccording to a tree structure is referred to as ‘the video encodingapparatus 100’.

The maximum coding unit splitter 110 may split a current picture basedon a maximum coding unit for the current picture of an image. If thecurrent picture is larger than the maximum coding unit, image data ofthe current picture may be split into the at least one maximum codingunit. The maximum coding unit according to an exemplary embodiment maybe a data unit having a size of 32×32, 64×64, 128×128, 256×256, etc.,wherein a shape of the data unit is a square having a width and lengthin squares of 2. The image data may be output to the coding unitdeterminer 120 according to the at least one maximum coding unit.

A coding unit according to an exemplary embodiment may be characterizedby a maximum size and a depth. The depth denotes a number of times thecoding unit is spatially split from the maximum coding unit, and as thedepth increases, deeper encoding units according to depths may be splitfrom the maximum coding unit to a minimum coding unit. A depth of themaximum coding unit is an uppermost depth and a depth of the minimumcoding unit is a lowermost depth. Since a size of a coding unitcorresponding to each depth decreases as the depth of the maximum codingunit increases, a coding unit corresponding to an upper depth mayinclude a plurality of coding units corresponding to lower depths.

As described above, the image data of the current picture is split intothe maximum coding units according to a maximum size of the coding unit,and each of the maximum coding units may include deeper coding unitsthat are split according to depths. Since the maximum coding unitaccording to an exemplary embodiment is split according to depths, theimage data of a spatial domain included in the maximum coding unit maybe hierarchically classified according to depths.

A maximum depth and a maximum size of a coding unit, which limit thetotal number of times a height and a width of the maximum coding unitare hierarchically split may be predetermined.

The coding unit determiner 120 encodes at least one split regionobtained by splitting a region of the maximum coding unit according todepths, and determines a depth to output a finally encoded image dataaccording to the at least one split region. In other words, the codingunit determiner 120 determines a coded depth by encoding the image datain the deeper coding units according to depths, according to the maximumcoding unit of the current picture, and selecting a depth having theleast encoding error. Thus, the encoded image data of the coding unitcorresponding to the determined coded depth is finally output. Also, thecoding units corresponding to the coded depth may be regarded as encodedcoding units. The determined coded depth and the encoded image dataaccording to the determined coded depth are output to the output unit130.

The image data in the maximum coding unit is encoded based on the deepercoding units corresponding to at least one depth equal to or below themaximum depth, and results of encoding the image data are compared basedon each of the deeper coding units. A depth having the least encodingerror may be selected after comparing encoding errors of the deepercoding units. At least one coded depth may be selected for each maximumcoding unit.

The size of the maximum coding unit is split as a coding unit ishierarchically split according to depths, and as the number of codingunits increases. Also, even if coding units correspond to the same depthin one maximum coding unit, it is determined whether to split each ofthe coding units corresponding to the same depth to a lower depth bymeasuring an encoding error of the image data of the each coding unit,separately. Accordingly, even when image data is included in one maximumcoding unit, the image data is split into regions according to thedepths and the encoding errors may differ according to regions in theone maximum coding unit, and thus the coded depths may differ accordingto regions in the image data. Thus, one or more coded depths may bedetermined in one maximum coding unit, and the image data of the maximumcoding unit may be divided according to coding units of at least onecoded depth.

Accordingly, the coding unit determiner 120 may determine coding unitshaving a tree structure included in the maximum coding unit. The ‘codingunits having a tree structure’ according to an exemplary embodimentinclude coding units corresponding to a depth determined to be the codeddepth, from among all deeper coding units included in the maximum codingunit. A coding unit of a coded depth may be hierarchically determinedaccording to depths in the same region of the maximum coding unit, andmay be independently determined in different regions. Similarly, a codeddepth in a current region may be independently determined from a codeddepth in another region.

A maximum depth according to an exemplary embodiment is an index relatedto the number of times splitting is performed from a maximum coding unitto a minimum coding unit. A first maximum depth according to anexemplary embodiment may denote the total number of times splitting isperformed from the maximum coding unit to the minimum coding unit. Asecond maximum depth according to an exemplary embodiment may denote thetotal number of depth levels from the maximum coding unit to the minimumcoding unit. For example, when a depth of the maximum coding unit is 0,a depth of a coding unit, in which the maximum coding unit is splitonce, may be set to 1, and a depth of a coding unit, in which themaximum coding unit is split twice, may be set to 2. Here, if theminimum coding unit is a coding unit in which the maximum coding unit issplit four times, 5 depth levels of depths 0, 1, 2, 3 and 4 exist, andthus the first maximum depth may be set to 4, and the second maximumdepth may be set to 5.

Prediction encoding and transformation may be performed according to themaximum coding unit. The prediction encoding and the transformation arealso performed based on the deeper coding units according to a depthequal to or depths less than the maximum depth, according to the maximumcoding unit. Transformation may be performed according to a method oforthogonal transformation or integer transformation.

Since the number of deeper coding units increases whenever the maximumcoding unit is split according to depths, encoding including theprediction encoding and the transformation is performed on all of thedeeper coding units generated as the depth increases. For convenience ofdescription, the prediction encoding and the transformation will now bedescribed based on a coding unit of a current depth, in a maximum codingunit.

The video encoding apparatus 100 may variously select a size or shape ofa data unit for encoding the image data. In order to encode the imagedata, operations, such as prediction encoding, transformation, andentropy encoding, are performed, and at this time, the same data unitmay be used for all operations or different data units may be used foreach operation.

For example, the video encoding apparatus 100 may select not only acoding unit for encoding the image data, but also a data unit differentfrom the coding unit so as to perform the prediction encoding on theimage data in the coding unit. In order to perform prediction encodingon the maximum coding unit, the prediction encoding may be performedbased on a coding unit corresponding to a coded depth, i.e., based on acoding unit that is no longer split into coding units corresponding to alower depth. Hereinafter, the coding unit that is no longer split andbecomes a basis unit for prediction encoding will now be referred to asa ‘prediction unit’. A partition obtained by splitting the predictionunit may include a prediction unit or a data unit obtained by splittingat least one of a height and a width of the prediction unit. Thepartition is a data unit obtained by dividing the prediction unit of thecoding unit and the prediction unit may be a partition having the samesize as the coding unit.

For example, when a coding unit of 2N×2N (where N is a positive integer)is no longer split and becomes a prediction unit of 2N×2N, a size of apartition may be 2N×2N, 2N×N, N×2N, or N×N. Examples of a partition typeinclude symmetrical partitions that are obtained by symmetricallysplitting a height or width of the prediction unit, partitions obtainedby asymmetrically splitting the height or width of the prediction unit,such as 1:n or n:1, partitions that are obtained by geometricallysplitting the prediction unit, and partitions having arbitrary shapes.

A prediction mode of the prediction unit may be at least one of an intramode, a inter mode, and a skip mode. For example, the intra mode or theinter mode may be performed on the partition of 2N×2N, 2N×N, N×2N, orN×N. Also, the skip mode may be performed only on the partition of2N×2N. The encoding is independently performed on one prediction unit ina coding unit, thereby selecting a prediction mode having a leastencoding error.

The video encoding apparatus 100 may also perform the transformation onthe image data in a coding unit based not only on the coding unit forencoding the image data, but also based on a transformation unit that isdifferent from the coding unit. In order to perform the transformationin the coding unit, the transformation may be performed based on a dataunit having a size smaller than or equal to the coding unit. Forexample, the transformation unit for the transformation may include atransformation unit for an intra mode and a data unit for an inter mode.

Similarly to the coding unit according to the tree structure accordingto the present embodiment, the transformation unit in the coding unitmay be recursively split into smaller sized regions and residual data inthe coding unit may be divided according to the transformation havingthe tree structure according to transformation depths.

According to an exemplary embodiment, a transformation depth indicatingthe number of times splitting is performed to reach the transformationunit by splitting the height and width of the coding unit may also beset in the transformation unit. For example, when the size of atransformation unit of a current coding unit is 2N×2N, a transformationdepth may be set to 0. When the size of a transformation unit is N×N,the transformation depth may be set to 1. In addition, when the size ofthe transformation unit is N/2×N/2, the transformation depth may be setto 2. That is, the transformation unit according to the tree structuremay also be set according to the transformation depth.

Encoding information according to coding units corresponding to a codeddepth requires not only information about the coded depth, but alsoabout information related to prediction encoding and transformation.Accordingly, the coding unit determiner 120 not only determines a codeddepth having a least encoding error, but also determines a partitiontype in a prediction unit, a prediction mode according to predictionunits, and a size of a transformation unit for transformation.

Coding units and a prediction unit/partition according to a treestructure in a maximum coding unit, and a method of determining atransformation unit, according to exemplary embodiments, will bedescribed in detail later with reference to FIGS. 7 through 19.

The coding unit determiner 120 may measure an encoding error of deepercoding units according to depths by using Rate-Distortion Optimizationbased on Lagrangian multipliers.

The output unit 130 outputs the image data of the maximum coding unit,which is encoded based on the at least one coded depth determined by thecoding unit determiner 120, and information about the encoding modeaccording to the coded depth, in bitstreams.

The encoded image data may be obtained by encoding residual data of animage.

The information about the encoding mode according to the coded depth mayinclude information about the coded depth, the partition type in theprediction unit, the prediction mode, and the size of the transformationunit.

The information about the coded depth may be defined by using splitinformation according to depths, which indicates whether encoding isperformed on coding units of a lower depth instead of a current depth.If the current depth of the current coding unit is the coded depth,image data in the current coding unit is encoded and output, and thusthe split information may be defined not to split the current codingunit to a lower depth. Alternatively, if the current depth of thecurrent coding unit is not the coded depth, the encoding is performed onthe coding unit of the lower depth, and thus the split information maybe defined to split the current coding unit to obtain the coding unitsof the lower depth.

If the current depth is not the coded depth, encoding is performed onthe coding unit that is split into the coding unit of the lower depth.Since at least one coding unit of the lower depth exists in one codingunit of the current depth, the encoding is repeatedly performed on eachcoding unit of the lower depth, and thus the encoding may be recursivelyperformed for the coding units having the same depth.

Since the coding units having a tree structure are determined for onemaximum coding unit, and information about at least one encoding mode isdetermined for a coding unit of a coded depth, information about atleast one encoding mode may be determined for one maximum coding unit.Also, a coded depth of the image data of the maximum coding unit may bedifferent according to locations since the image data is hierarchicallysplit according to depths, and thus information about the coded depthand the encoding mode may be set for the image data.

Accordingly, the output unit 130 may assign encoding information about acorresponding coded depth and an encoding mode to at least one of thecoding unit, the prediction unit, and a minimum unit included in themaximum coding unit.

The minimum unit according to an exemplary embodiment is a rectangulardata unit obtained by splitting the minimum coding unit constituting thelowermost depth by 4. Alternatively, the minimum unit may be a maximumrectangular data unit having a maximum size, which is included in all ofthe coding units, prediction units, partition units, and transformationunits included in the maximum coding unit.

For example, the encoding information output through the output unit 130may be classified into encoding information according to coding units,and encoding information according to prediction units. The encodinginformation according to the coding units may include the informationabout the prediction mode and about the size of the partitions. Theencoding information according to the prediction units may includeinformation about an estimated direction of an inter mode, about areference image index of the inter mode, about a motion vector, about achroma component of an intra mode, and about an interpolation method ofthe intra mode.

Also, information about a maximum size of the coding unit definedaccording to pictures, slices, or GOPs, and information about a maximumdepth may be inserted into a header of a bitstream, a SPS (SequenceParameter Set) or a picture parameter set (PPS).

In addition, information about a maximum size of a transformation unitand information about a minimum size of a transformation, which areacceptable for a current video may also be output via a header of abitstream, a SPS or a PPS. The output unit 130 may encode and outputreference information, prediction information, single-directionprediction information, and information about a slice type including afourth slice type, which are related to prediction described withreference to FIGS. 1 through 6.

In the video encoding apparatus 100, the deeper coding unit may be acoding unit obtained by dividing a height or width of a coding unit ofan upper depth, which is one layer above, by two. In other words, whenthe size of the coding unit of the current depth is 2N×2N, the size ofthe coding unit of the lower depth is N×N. Also, the coding unit of thecurrent depth having the size of 2N×2N may include a maximum value 4 ofthe coding unit of the lower depth.

Accordingly, the video encoding apparatus 100 may form the coding unitshaving the tree structure by determining coding units having an optimumshape and an optimum size for each maximum coding unit, based on thesize of the maximum coding unit and the maximum depth determinedconsidering characteristics of the current picture. Also, since encodingmay be performed on each maximum coding unit by using any one of variousprediction modes and transformations, an optimum encoding mode may bedetermined considering characteristics of the coding unit of variousimage sizes.

Thus, if an image having high resolution or large data amount is encodedin a related macroblock, a number of macroblocks per picture excessivelyincreases. Accordingly, a number of pieces of compressed informationgenerated for each macroblock increases, and thus it is difficult totransmit the compressed information and data compression efficiencydecreases. However, by using the video encoding apparatus 100, imagecompression efficiency may be increased since a coding unit is adjustedwhile considering characteristics of an image while increasing a maximumsize of a coding unit while considering a size of the image.

The video encoding apparatus 100 of FIG. 7 may perform the operation ofthe inter prediction apparatus 10 as described with reference to FIG. 1.

The coding unit determiner 120 may perform an operation of the interprediction apparatus 10. For each maximum coding unit, a prediction unitfor inter prediction may be determined in coding units according to atree structure and inter prediction may be performed in predictionunits.

In particular, whether a first reference list from among reference listsof a collocated block of a current block is referred to ispreferentially checked, for inter prediction of a current predictionunit in a prediction mode. The first reference list may include imagesthat are positioned in an opposite direction to a direction from thecurrent image to the collocated block.

When the first reference list is capable of being referred to, it doesnot have to be checked whether a second reference list is referred to.When the first reference list is not referred to, whether the secondreference list is referred to may be checked. Based on a result of thechecking, a single collocated reference list may be determined fromamong the first reference list and the second reference list and areference block of a current prediction unit may be determined based onmotion information of the collocated reference list. Inter predictionmay be performed on the current prediction unit by using the referenceblock of the current prediction unit to generate a residual value. Aparameter ‘collocated_from_10_flag’ or a parameter ‘colDir’ indicating acollocated block of the current prediction unit may be transmitted.

FIG. 8 is a block diagram of a video decoding apparatus 200 based on acoding unit according to a tree structure, according to an exemplaryembodiment.

The video decoding apparatus 200 based on the coding unit according tothe tree structure includes a receiver 210, an image data and encodinginformation extractor 220, and an image data decoder 230. Hereinafter,for convenience of description, the video decoding apparatus 200 usingvideo prediction based on a coding unit according to a tree structurewill be referred to as the ‘video decoding apparatus 200’.

Definitions of various terms, such as a coding unit, a depth, aprediction unit, a transformation unit, and information about variousencoding modes, for decoding operations of the video decoding apparatus200 are identical to those described with reference to FIG. 7 and thevideo encoding apparatus 100.

The receiver 210 receives and parses a bitstream of an encoded video.The image data and encoding information extractor 220 extracts encodedimage data for each coding unit from the parsed bitstream, wherein thecoding units have a tree structure according to each maximum codingunit, and outputs the extracted image data to the image data decoder230. The image data and encoding information extractor 220 may extractinformation about a maximum size of a coding unit of a current picture,from a header about the current picture, a SPS, or a PPS.

Also, the image data and encoding information extractor 220 extractsinformation about a coded depth and an encoding mode for the codingunits having a tree structure according to each maximum coding unit,from the parsed bitstream. The extracted information about the codeddepth and the encoding mode is output to the image data decoder 230. Inother words, the image data in a bit stream is split into the maximumcoding unit so that the image data decoder 230 decodes the image datafor each maximum coding unit.

The information about the coded depth and the encoding mode according tothe maximum coding unit may be set for information about at least onecoding unit corresponding to the coded depth, and information about anencoding mode may include information about a partition type of acorresponding coding unit corresponding to the coded depth, about aprediction mode, and a size of a transformation unit. Also, splittinginformation according to depths may be extracted as the informationabout the coded depth.

The information about the coded depth and the encoding mode according toeach maximum coding unit extracted by the image data and encodinginformation extractor 220 is information about a coded depth and anencoding mode determined to generate a minimum encoding error when anencoder, such as the video encoding apparatus 100, repeatedly performsencoding for each deeper coding unit according to depths according toeach maximum coding unit. Accordingly, the video decoding apparatus 200may restore an image by decoding the image data according to a codeddepth and an encoding mode that generates the minimum encoding error.

Since encoding information about the coded depth and the encoding modemay be assigned to a predetermined data unit from among a correspondingcoding unit, a prediction unit, and a minimum unit, the image data andencoding information extractor 220 may extract the information about thecoded depth and the encoding mode according to the predetermined dataunits. The predetermined data units to which the same information aboutthe coded depth and the encoding mode is assigned may be inferred to bethe data units included in the same maximum coding unit.

The image data decoder 230 restores the current picture by decoding theimage data in each maximum coding unit based on the information aboutthe coded depth and the encoding mode according to the maximum codingunits. In other words, the image data decoder 230 may decode the encodedimage data based on the extracted information about the partition type,the prediction mode, and the transformation unit for each coding unitfrom among the coding units having the tree structure included in eachmaximum coding unit. A decoding process may include prediction includingintra prediction and motion compensation, and inverse transformation.Inverse transformation may be performed according to a method of inverseorthogonal transformation or inverse integer transformation.

The image data decoder 230 may perform intra prediction or motioncompensation according to a partition and a prediction mode of eachcoding unit, based on the information about the partition type and theprediction mode of the prediction unit of the coding unit according tocoded depths.

In addition, the image data decoder 230 may read transformation unitinformation according to a tree structure for each coding unit so as todetermine transform units for each coding unit and perform inversetransformation based on a transformation unit for each coding unit, forinverse transformation for each maximum coding unit. Via the inversetransformation, a pixel value of a spatial region of the coding unit maybe restored.

The image data decoder 230 may determine at least one coded depth of acurrent maximum coding unit by using split information according todepths. If the split information indicates that image data is no longersplit in the current depth, the current depth is a coded depth.Accordingly, the image data decoder 230 may decode encoded data of atleast one coding unit corresponding to each coded depth in the currentmaximum coding unit by using the information about the partition type ofthe prediction unit, the prediction mode, and the size of thetransformation unit for each coding unit corresponding to the codeddepth, and output the image data of the current maximum coding unit.

In other words, data units containing the encoding information includingthe same split information may be gathered by observing the encodinginformation set assigned for the predetermined data unit from among thecoding unit, the prediction unit, and the minimum unit, and the gathereddata units may be considered to be one data unit to be decoded by theimage data decoder 230 in the same encoding mode. For each coding unitdetermined as described above, information about an encoding mode may beobtained so as to decode the current coding unit.

The image data decoder 230 of the video decoding apparatus 200 of FIG. 8may perform the operation of the inter prediction apparatus 10 asdescribed above with reference to FIG. 1.

The image data decoder 230 may determine a prediction unit for interprediction for each coding unit according to a tree structure and mayperform inter prediction for each prediction unit, for a maximum codingunit.

In particular, a collocated block of a current block is determined fromamong restored samples, for inter prediction of a current block in aninter mode. A collocated block of a current prediction unit may bedetermined based on a parameter ‘collocated_from_10_flag’ or a parameter‘colDir’ that is a current prediction unit obtained by parsing abitstream.

Whether a first reference list is referred to from among reference listsof the collocated block is preferentially checked. The first referencelist may include images that are positioned in and opposite direction toa direction from the current image to the collocated block.

When the first reference list is capable of being referred to, it doesnot have to be checked whether a second reference list is referred to.When the first reference list is not referred to, whether the secondreference list is referred to may be checked. Based on a result of thechecking, a single collocated reference list may be determined fromamong the first reference list and the second reference list and areference block of the current prediction unit may be determined basedon motion information of the collocated reference list. Motioncompensation may be performed on the current prediction unit by usingthe reference block of the current prediction unit to generate a blockpixel sample value.

In addition, when an image is restored in a low-delay condition, areference image may be determined according to a reference list of thecurrent prediction unit regardless of the collocated reference list.

The video decoding apparatus 200 may obtain information about at leastone coding unit that generates the minimum encoding error when encodingis recursively performed for each maximum coding unit, and may use theinformation to decode the current picture. In other words, the codingunits having the tree structure determined to be the optimum codingunits in each maximum coding unit may be decoded. Also, the maximum sizeof a coding unit is determined considering resolution and an amount ofimage data.

Accordingly, even if image data has high resolution and a large amountof data, the image data may be efficiently decoded and restored by usinga size of a coding unit and an encoding mode, which are adaptivelydetermined according to characteristics of the image data, by usinginformation about an optimum encoding mode received from an encoder.

FIG. 9 is a diagram for describing a concept of coding units accordingto an exemplary embodiment.

A size of a coding unit may be expressed in width×height, and may be64×64, 32×32, 16×16, and 8×8. A coding unit of 64×64 may be split intopartitions of 64×64, 64×32, 32×64, or 32×32, and a coding unit of 32×32may be split into partitions of 32×32, 32×16, 16×32, or 16×16, a codingunit of 16×16 may be split into partitions of 16×16, 16×8, 8×16, or 8×8,and a coding unit of 8×8 may be split into partitions of 8×8, 8×4, 4×8,or 4×4.

In video data 310, a resolution is 1920×1080, a maximum size of a codingunit is 64, and a maximum depth is 2. In video data 320, a resolution is1920×1080, a maximum size of a coding unit is 64, and a maximum depth is3. In video data 330, a resolution is 352×288, a maximum size of acoding unit is 16, and a maximum depth is 1. The maximum depth shown inFIG. 9 denotes a total number of splits from a maximum coding unit to aminimum decoding unit.

If a resolution is high or a data amount is large, a maximum size of acoding unit may be large so as to not only increase encoding efficiencybut also to accurately reflect characteristics of an image. Accordingly,the maximum size of the coding unit of the video data 310 and 320 havingthe higher resolution than the video data 330 may be 64.

Since the maximum depth of the video data 310 is 2, coding units 315 ofthe video data 310 may include a maximum coding unit having a long axissize of 64, and coding units having long axis sizes of 32 and 16 sincedepths are deepened to two layers by splitting the maximum coding unittwice. Meanwhile, since the maximum depth of the video data 330 is 1,coding units 335 of the video data 330 may include a maximum coding unithaving a long axis size of 16, and coding units having a long axis sizeof 8 since depths are deepened to one layer by splitting the maximumcoding unit once.

Since the maximum depth of the video data 320 is 3, coding units 325 ofthe video data 320 may include a maximum coding unit having a long axissize of 64, and coding units having long axis sizes of 32, 16, and 8since the depths are deepened to 3 layers by splitting the maximumcoding unit three times. As a depth increases, detailed information maybe precisely expressed.

FIG. 10 is a block diagram of an image encoder 400 based on codingunits, according to an exemplary embodiment.

The image encoder 400 performs operations of the coding unit determiner120 of the video encoding apparatus 100 to encode image data. In otherwords, an intra predictor 410 performs intra prediction on coding unitsin an intra mode, from among a current frame 405, and a motion estimator420 and a motion compensator 425 performs inter estimation and motioncompensation on coding units in an inter mode from among the currentframe 405 by using the current frame 405, and a reference frame 495.

Data output from the intra predictor 410, the motion estimator 420, andthe motion compensator 425 is output as a quantized transformationcoefficient through a transformer 430 and a quantizer 440. The quantizedtransformation coefficient is restored as data in a spatial domainthrough an inverse quantizer 460 and an inverse transformer 470, and therestored data in the spatial domain is output as the reference frame 495after being post-processed through a deblocking unit 480 (e.g., adeblocker) and a loop filtering unit 490 (e.g., a loop filterer). Thequantized transformation coefficient may be output as a bitstream 455through an entropy encoder 450.

In order for the image encoder 400 to be applied in the video encodingapparatus 100, all elements of the image encoder 400, i.e., the intrapredictor 410, the motion estimator 420, the motion compensator 425, thetransformer 430, the quantizer 440, the entropy encoder 450, the inversequantizer 460, the inverse transformer 470, the deblocking unit 480, andthe loop filtering unit 490 perform operations based on each coding unitfrom among coding units having a tree structure while considering themaximum depth of each maximum coding unit.

Specifically, the intra predictor 410, the motion estimator 420, and themotion compensator 425 determines partitions and a prediction mode ofeach coding unit from among the coding units having a tree structurewhile considering the maximum size and the maximum depth of a currentmaximum coding unit, and the transformer 430 determines the size of thetransformation unit in each coding unit from among the coding unitshaving a tree structure.

In particular, in order to determine a reference image for interprediction of a current prediction unit, the motion compensator 425preferentially checks whether a first reference list of a collocatedblock is referred to, and does not recheck whether the remainingreference lists of the collocated block are referred to when motioninformation of the first reference list exists since the first referencelist of the collocated block is preferentially referred to. However,when the motion information of the first reference list does not existsince the first reference list of the collocated block is not referredto, the motion compensator 425 may recheck whether the remainingreference lists of the collocated block are referred to. The motioncompensator 425 may determine a reference list of the current predictionunit by using the reference list of the collocated block on which thecheck operation has been performed.

FIG. 11 is a block diagram of an image decoder 500 based on codingunits, according to an exemplary embodiment.

A parser 510 parses encoded image data to be decoded and informationabout encoding required for decoding from a bitstream 505. The encodedimage data is output as inverse quantized data through an entropydecoder 520 and an inverse quantizer 530, and the inverse quantized datais restored to image data in a spatial domain through an inversetransformer 540.

An intra predictor 550 performs intra prediction on coding units in anintra mode with respect to the image data in the spatial domain, and amotion compensator 560 performs motion compensation on coding units inan inter mode by using a reference frame 585.

The image data in the spatial domain, which passed through the intrapredictor 550 and the motion compensator 560, may be output as arestored frame 595 after being post-processed through a deblocking unit570 (e.g., a deblocker) and a loop filtering unit 580 (e.g., a loopfilterer). Also, the image data that is post-processed through thedeblocking unit 570 and the loop filtering unit 580 may be output as thereference frame 585.

In order to decode the image data in the image data decoder 230 of thevideo decoding apparatus 200, the image decoder 500 may performoperations that are performed after the parser 510 performs anoperation.

In order for the image decoder 500 to be applied in the video decodingapparatus 200, all elements of the image decoder 500, i.e., the parser510, the entropy decoder 520, the inverse quantizer 530, the inversetransformer 540, the intra predictor 550, the motion compensator 560,the deblocking unit 570, and the loop filtering unit 580 performoperations based on coding units having a tree structure for eachmaximum coding unit.

Specifically, the intra predictor 550 and the motion compensator 560perform operations based on partitions and a prediction mode for each ofthe coding units having a tree structure, and the inverse transformer540 perform operations based on a size of a transformation unit for eachcoding unit.

In particular, in order to determine a reference image for interprediction of a current prediction unit, the motion compensator 560preferentially checks whether a first reference list of a collocatedblock is referred to, and does not recheck whether the remainingreference lists of the collocated block are referred to when motioninformation of the first reference list exists since the first referencelist of the collocated block is preferentially referred to. However,when the motion information of the first reference list does not existsince the first reference list of the collocated block is not referredto, the motion compensator 560 may recheck whether the remainingreference lists of the collocated block are referred to. The motioncompensator 560 may determine a reference list of the current predictionunit by using the reference list of the collocated block on which thecheck operation has been performed.

FIG. 12 is a diagram illustrating deeper coding units according todepths, and partitions, according to an exemplary embodiment.

The video encoding apparatus 100 and the video decoding apparatus 200use hierarchical coding units so as to consider characteristics of animage. A maximum height, a maximum width, and a maximum depth of codingunits may be adaptively determined according to the characteristics ofthe image, or may be differently set by a user. Sizes of deeper codingunits according to depths may be determined according to thepredetermined maximum size of the coding unit.

In a hierarchical structure 600 of coding units, according to anexemplary embodiment, the maximum height and the maximum width of thecoding units are each 64, and the maximum depth is 4. In this case, themaximum depth refers to a total number of times the coding unit is splitfrom the maximum coding unit to the minimum coding unit. Since a depthincreases along a vertical axis of the hierarchical structure 600, aheight and a width of the deeper coding unit are each split. Also, aprediction unit and partitions, which are bases for prediction encodingof each deeper coding unit, are shown along a horizontal axis of thehierarchical structure 600.

In other words, a coding unit 610 is a maximum coding unit in thehierarchical structure 600, wherein a depth is 0 and a size, i.e., aheight by width, is 64×64. The depth increases along the vertical axis,and a coding unit 620 having a size of 32×32 and a depth of 1, a codingunit 630 having a size of 16×16 and a depth of 2, and a coding unit 640having a size of 8×8 and a depth of 3 exist. The coding unit 640 havingthe size of 8×8 and the depth of 3 is a minimum coding unit.

The prediction unit and the partitions of a coding unit are arrangedalong the horizontal axis according to each depth. In other words, ifthe coding unit 610 having the size of 64×64 and the depth of 0 is aprediction unit, the prediction unit may be split into partitionsincluded in the encoding unit 610, i.e. a partition 610 having a size of64×64, partitions 612 having the size of 64×32, partitions 614 havingthe size of 32×64, or partitions 616 having the size of 32×32.

Similarly, a prediction unit of the coding unit 620 having the size of32×32 and the depth of 1 may be split into partitions included in thecoding unit 620, i.e. a partition 620 having a size of 32×32, partitions622 having a size of 32×16, partitions 624 having a size of 16×32, andpartitions 626 having a size of 16×16.

Similarly, a prediction unit of the coding unit 630 having the size of16×16 and the depth of 2 may be split into partitions included in thecoding unit 630, i.e. a partition having a size of 16×16 included in thecoding unit 630, partitions 632 having a size of 16×8, partitions 634having a size of 8×16, and partitions 636 having a size of 8×8.

Similarly, a prediction unit of the coding unit 640 having the size of8×8 and the depth of 3 may be split into partitions included in thecoding unit 640, i.e. a partition having a size of 8×8 included in thecoding unit 640, partitions 642 having a size of 8×4, partitions 644having a size of 4×8, and partitions 646 having a size of 4×4.

In order to determine the at least one coded depth of the coding unitsconstituting the maximum coding unit 610, the coding unit determiner 120of the video encoding apparatus 100 performs encoding for coding unitscorresponding to each depth included in the maximum coding unit 610.

A number of deeper coding units according to depths including data inthe same range and the same size increases as the depth increases. Forexample, four coding units corresponding to a depth of 2 are required tocover data that is included in one coding unit corresponding to a depthof 1. Accordingly, in order to compare encoding results of the same dataaccording to depths, the coding unit corresponding to the depth of 1 andfour coding units corresponding to the depth of 2 are each encoded.

In order to perform encoding for a current depth from among the depths,a least encoding error may be selected for the current depth byperforming encoding for each prediction unit in the coding unitscorresponding to the current depth, along the horizontal axis of thehierarchical structure 600. Alternatively, the minimum encoding errormay be searched for by comparing the least encoding errors according todepths, by performing encoding for each depth as the depth increasesalong the vertical axis of the hierarchical structure 600. A depth and apartition having the minimum encoding error in the coding unit 610 maybe selected as the coded depth and a partition type of the coding unit610.

FIG. 13 is a diagram for describing a relationship between a coding unit710 and transformation units 720, according to an exemplary embodiment.

The video encoding apparatus 100 or 200 encodes or decodes an imageaccording to coding units having sizes smaller than or equal to amaximum coding unit for each maximum coding unit. Sizes oftransformation units for transformation during encoding may be selectedbased on data units that are not larger than a corresponding codingunit.

For example, in the video encoding apparatus 100 or 200, if a size ofthe coding unit 710 is 64×64, transformation may be performed by usingthe transformation units 720 having a size of 32×32.

Also, data of the coding unit 710 having the size of 64×64 may beencoded by performing the transformation on each of the transformationunits having the size of 32×32, 16×16, 8×8, and 4×4, which are smallerthan 64×64, and then a transformation unit having the least coding errormay be selected.

FIG. 14 is a diagram for describing encoding information of coding unitscorresponding to a coded depth, according to an exemplary embodiment.

The output unit 130 of the video encoding apparatus 100 may encode andtransmit information 800 about a partition type, information 810 about aprediction mode, and information 820 about a size of a transformationunit for each coding unit corresponding to a coded depth, as informationabout an encoding mode.

The information 800 indicates information about a shape of a partitionobtained by splitting a prediction unit of a current coding unit,wherein the partition is a data unit for prediction encoding the currentcoding unit. For example, a current coding unit CU_0 having a size of2N×2N may be split into any one of a partition 802 having a size of2N×2N, a partition 804 having a size of 2N×N, a partition 806 having asize of N×2N, and a partition 808 having a size of N×N. Here, theinformation 800 about a partition type is set to indicate one of thepartition 804 having a size of 2N×N, the partition 806 having a size ofN×2N, and the partition 808 having a size of N×N

The information 810 indicates a prediction mode of each partition. Forexample, the information 810 may indicate a mode of prediction encodingperformed on a partition indicated by the information 800, i.e., anintra mode 812, an inter mode 814, or a skip mode 816.

The information 820 indicates a transformation unit to be based on whentransformation is performed on a current coding unit. For example, thetransformation unit may be a first intra transformation unit 822, asecond intra transformation unit 824, a first inter transformation unit826, or a second inter transformation unit 828.

The image data and encoding information extractor 220 of the videodecoding apparatus 200 may extract and use the information 800, 810, and820 for decoding, according to each deeper coding unit.

FIG. 15 is a diagram of deeper coding units according to depths,according to an exemplary embodiment.

Split information may be used to indicate a change of a depth. The spiltinformation indicates whether a coding unit of a current depth is splitinto coding units of a lower depth.

A prediction unit 910 for prediction encoding a coding unit 900 having adepth of 0 and a size of 2N_0×2N_0 may include partitions of a partitiontype 912 having a size of 2N_0×2N_0, a partition type 914 having a sizeof 2N_0×N_0, a partition type 916 having a size of N_0×2N_0, and apartition type 918 having a size of N_0×N_0. FIG. 15 only illustratesthe partition types 912 through 918 which are obtained by symmetricallysplitting the prediction unit 910, but a partition type is not limitedthereto, and the partitions of the prediction unit 910 may includeasymmetrical partitions, partitions having a predetermined shape, andpartitions having a geometrical shape.

Prediction encoding is repeatedly performed on one partition having asize of 2N_0×2N_0, two partitions having a size of 2N_0×N_0, twopartitions having a size of N_0×2N_0, and four partitions having a sizeof N_0×N_0, according to each partition type. The prediction encoding inan intra mode and an inter mode may be performed on the partitionshaving the sizes of 2N_0×2N_0, N_0×2N_0, 2N_0×N_0, and N_0×N_0. Theprediction encoding in a skip mode is performed only on the partitionhaving the size of 2N_0×2N_0.

Errors of encoding including the prediction encoding in the partitiontypes 912 through 918 are compared, and the least encoding error isdetermined among the partition types. If an encoding error is smallestin one of the partition types 912 through 916, the prediction unit 910may not be split into a lower depth.

If the encoding error is the smallest in the partition type 918, a depthis changed from 0 to 1 to split the partition type 918 in operation 920,and encoding is repeatedly performed on coding units 930 having a depthof 2 and a size of N_0×N_0 to search for a minimum encoding error.

A prediction unit 940 for prediction encoding the coding unit 930 havinga depth of 1 and a size of 2N_1×2N_1 (=N_0×N_0) may include partitionsof a partition type 942 having a size of 2N_1×2N_1, a partition type 944having a size of 2N_1×N_1, a partition type 946 having a size ofN_1×2N_1, and a partition type 948 having a size of N_1×N_1.

If an encoding error is the smallest in the partition type 948, a depthis changed from 1 to 2 to split the partition type 948 in operation 950,and encoding is repeatedly performed on coding units 960, which have adepth of 2 and a size of N_2×N_2 to search for a minimum encoding error.

When a maximum depth is d, split operation according to each depth maybe performed up to when a depth becomes d−1, and split information maybe encoded as up to when a depth is one of 0 to d−2. In other words,when encoding is performed up to when the depth is d−1 after a codingunit corresponding to a depth of d−2 is split in operation 970, aprediction unit 990 for prediction encoding a coding unit 980 having adepth of d−1 and a size of 2N_(d−1)×2N_(d−1) may include partitions of apartition type 992 having a size of 2N_(d−1)×2N_(d−1), a partition type994 having a size of 2N_(d−1)×N_(d−1), a partition type 996 having asize of N_(d−1)×2N_(d−1), and a partition type 998 having a size ofN_(d−1)×N_(d−1).

Prediction encoding may be repeatedly performed on one partition havinga size of 2N_(d−1)×2N_(d−1), two partitions having a size of2N_(d−1)×N_(d−1), two partitions having a size of N_(d−1)×2N_(d−1), fourpartitions having a size of N_(d−1)×N_(d−1) from among the partitiontypes 992 through 998 to search for a partition type having a minimumencoding error.

Even when the partition type 998 has the minimum encoding error, since amaximum depth is d, a coding unit CU_(d−1) having a depth of d−1 is nolonger split to a lower depth, and a coded depth for the coding unitsconstituting a current maximum coding unit 900 is determined to be d−1and a partition type of the current maximum coding unit 900 may bedetermined to be N_(d−1)×N (d−1). Also, since the maximum depth is d anda minimum coding unit 980 having a lowermost depth of d−1 is no longersplit to a lower depth, split information for the minimum coding unit980 is not set.

A data unit 999 may be a ‘minimum unit’ for the current maximum codingunit. A minimum unit according to an exemplary embodiment may be arectangular data unit obtained by splitting a minimum coding unit 980 by4. By performing the encoding repeatedly, the video encoding apparatus100 may select a depth having the least encoding error by comparingencoding errors according to depths of the coding unit 900 to determinea coded depth, and set a corresponding partition type and a predictionmode as an encoding mode of the coded depth.

As such, the minimum encoding errors according to depths are compared inall of the depths of 1 through d, and a depth having the least encodingerror may be determined as a coded depth. The coded depth, the partitiontype of the prediction unit, and the prediction mode may be encoded andtransmitted as information about an encoding mode. Also, since a codingunit is split from a depth of 0 to a coded depth, only split informationof the coded depth is set to 0, and split information of depthsexcluding the coded depth is set to 1.

The image data and encoding information extractor 220 of the videodecoding apparatus 200 may extract and use the information about thecoded depth and the prediction unit of the coding unit 900 to decode thepartition 912. The video decoding apparatus 200 may determine a depth,in which split information is 0, as a coded depth by using splitinformation according to depths, and use information about an encodingmode of the corresponding depth for decoding.

FIGS. 16 through 18 are diagrams for describing a relationship betweencoding units 1010, prediction units 1060, and transformation units 1070,according to an exemplary embodiment.

The coding units 1010 are coding units having a tree structure,corresponding to coded depths determined by the video encoding apparatus100, in a maximum coding unit. The prediction units 1060 are partitionsof prediction units of each of the coding units 1010, and thetransformation units 1070 are transformation units of each of the codingunits 1010.

When a depth of a maximum coding unit is 0 in the coding units 1010,depths of coding units 1012 and 1054 are 1, depths of coding units 1014,1016, 1018, 1028, 1050, and 1052 are 2, depths of coding units 1020,1022, 1024, 1026, 1030, 1032, and 1048 are 3, and depths of coding units1040, 1042, 1044, and 1046 are 4.

In the prediction units 1060, some encoding units 1014, 1016, 1022,1032, 1048, 1050, 1052, and 1054 are obtained by splitting the codingunits in the encoding units 1010. In other words, partition types in thecoding units 1014, 1022, 1050, and 1054 have a size of 2N×N, partitiontypes in the coding units 1016, 1048, and 1052 have a size of N×2N, anda partition type of the coding unit 1032 has a size of N×N. Predictionunits and partitions of the coding units 1010 are smaller than or equalto each coding unit.

Transformation or inverse transformation is performed on image data ofthe coding unit 1052 in the transformation units 1070 in a data unitthat is smaller than the coding unit 1052. Also, the coding units 1014,1016, 1022, 1032, 1048, 1050, and 1052 in the transformation units 1070are different from those in the prediction units 1060 in terms of sizesand shapes. In other words, the video encoding and decoding apparatuses100 and 200 may perform intra prediction, motion estimation, motioncompensation, transformation, and inverse transformation individually ona data unit in the same coding unit.

Accordingly, encoding is recursively performed on each of coding unitshaving a hierarchical structure in each region of a maximum coding unitto determine an optimum coding unit, and thus coding units having arecursive tree structure may be obtained. Encoding information mayinclude split information about a coding unit, information about apartition type, information about a prediction mode, and informationabout a size of a transformation unit. Table 1 shows the encodinginformation that may be set by the video encoding and decodingapparatuses 100 and 200.

TABLE 1 Split Information 0 Split (Encoding on Coding Unit having Sizeof 2N × 2N and Current Depth of d) Information 1 Prediction PartitionType Size of Transformation Unit Repeatedly Mode Encode IntraSymmetrical Asymmetrical Split Split Coding Inter Partition TypePartition Type Information 0 of Information 1 of Units SkipTransformation Unit Transformation Unit having (Only 2N × 2N 2N × nU 2N× 2N N × N Lower 2N × 2N) 2N × N 2N × nD (Symmetrical Type) Depth of N ×2N nL × 2N N/2 × N/2 d + 1 N × N nR × 2N (Asymmetrical Type)

The output unit 130 of the video encoding apparatus 100 may output theencoding information about the coding units having a tree structure, andthe image data and encoding information extractor 220 of the videodecoding apparatus 200 may extract the encoding information about thecoding units having a tree structure from a received bitstream.

Split information indicates whether a current coding unit is split intocoding units of a lower depth. If split information of a current depth dis 0, a depth, in which a current coding unit is no longer split into alower depth, is a coded depth, and thus information about a partitiontype, prediction mode, and a size of a transformation unit may bedefined for the coded depth. If the current coding unit is further splitaccording to the split information, encoding is independently performedon four split coding units of a lower depth.

A prediction mode may be one of an intra mode, an inter mode, and a skipmode. The intra mode and the inter mode may be defined in all partitiontypes, and the skip mode is defined only in a partition type having asize of 2N×2N.

The information about the partition type may indicate symmetricalpartition types having sizes of 2N×2N, 2N×N, N×2N, and N×N, which areobtained by symmetrically splitting a height or a width of a predictionunit, and asymmetrical partition types having sizes of 2N×nU, 2N×nD,nL×2N, and nR×2N, which are obtained by asymmetrically splitting theheight or width of the prediction unit. The asymmetrical partition typeshaving the sizes of 2N×nU and 2N×nD may be respectively obtained bysplitting the height of the prediction unit in 1:3 and 3:1, and theasymmetrical partition types having the sizes of nL×2N and nR×2N may berespectively obtained by splitting the width of the prediction unit in1:3 and 3:1

The size of the transformation unit may be set to be two types in theintra mode and two types in the inter mode. In other words, if splitinformation of the transformation unit is 0, the size of thetransformation unit may be 2N×2N, which is the size of the currentcoding unit. If split information of the transformation unit is 1, thetransformation units may be obtained by splitting the current codingunit. Also, if a partition type of the current coding unit having thesize of 2N×2N is a symmetrical partition type, a size of atransformation unit may be N×N, and if the partition type of the currentcoding unit is an asymmetrical partition type, the size of thetransformation unit may be N/2×N/2.

The encoding information about coding units having a tree structure mayinclude at least one of a coding unit corresponding to a coded depth, aprediction unit, and a minimum unit. The coding unit corresponding tothe coded depth may include at least one of a prediction unit and aminimum unit containing the same encoding information.

Accordingly, it is determined whether adjacent data units are includedin the same coding unit corresponding to the coded depth by comparingencoding information of the adjacent data units. Also, a correspondingcoding unit corresponding to a coded depth is determined by usingencoding information of a data unit, and thus a distribution of codeddepths in a maximum coding unit may be determined.

Accordingly, if a current coding unit is predicted based on encodinginformation of adjacent data units, encoding information of data unitsin deeper coding units adjacent to the current coding unit may bedirectly referred to and used.

Alternatively, if a current coding unit is predicted based on encodinginformation of adjacent data units, data units adjacent to the currentcoding unit are searched using encoded information of the data units,and the searched adjacent coding units may be referred to for predictingthe current coding unit.

FIG. 19 is a diagram for describing a relationship between a codingunit, a prediction unit or a partition, and a transformation unit,according to encoding mode information of Table 1.

A maximum coding unit 1300 includes coding units 1302, 1304, 1306, 1312,1314, 1316, and 1318 of coded depths. Here, since the coding unit 1318is a coding unit of a coded depth, split information may be set to 0.Information about a partition type of the coding unit 1318 having a sizeof 2N×2N may be set to be one of a partition type 1322 having a size of2N×2N, a partition type 1324 having a size of 2N×N, a partition type1326 having a size of N×2N, a partition type 1328 having a size of N×N,a partition type 1332 having a size of 2N×nU, a partition type 1334having a size of 2N×nD, a partition type 1336 having a size of nL×2N,and a partition type 1338 having a size of nR×2N.

Split information (TU (Transformation Unit)size flag) of atransformation unit is a type of a transformation index. The size of thetransformation unit corresponding to the transformation index may bechanged according to a prediction unit type or partition type of thecoding unit.

For example, when the partition type is set to be symmetrical, i.e. thepartition type 1322, 1324, 1326, or 1328, a transformation unit 1342having a size of 2N×2N is set if split information (TU size flag) of atransformation unit is 0, and a transformation unit 1344 having a sizeof N×N is set if a TU size flag is 1.

When the partition type is set to be asymmetrical, i.e., the partitiontype 1332, 1334, 1336, or 1338, a transformation unit 1352 having a sizeof 2N×2N is set if a TU size flag is 0, and a transformation unit 1354having a size of N/2×N/2 is set if a TU size flag is 1.

Referring to FIG. 19, the TU size flag is a flag having a value or 0 or1, but the TU size flag is not limited to 1 bit, and a transformationunit may be hierarchically split having a tree structure while the TUsize flag increases from 0. Split information (TU size flag) of atransformation unit may be an example of a transformation index.

In this case, the size of a transformation unit that has been actuallyused may be expressed by using a TU size flag of a transformation unit,according to an exemplary embodiment, together with a maximum size andminimum size of the transformation unit. According to an exemplaryembodiment, the video encoding apparatus 100 is capable of encodingmaximum transformation unit size information, minimum transformationunit size information, and a maximum TU size flag. The result ofencoding the maximum transformation unit size information, the minimumtransformation unit size information, and the maximum TU size flag maybe inserted into an SPS. According to an exemplary embodiment, the videodecoding apparatus 200 may decode video by using the maximumtransformation unit size information, the minimum transformation unitsize information, and the maximum TU size flag.

For example, (a) if the size of a current coding unit is 64×64 and amaximum transformation unit size is 32×32, (a-1) then the size of atransformation unit may be 32×32 when a TU size flag is 0, (a-2) may be16×16 when the TU size flag is 1, and (a-3) may be 8×8 when the TU sizeflag is 2.

As another example, (b) if the size of the current coding unit is 32×32and a minimum transformation unit size is 32×32, (b-1) then the size ofthe transformation unit may be 32×32 when the TU size flag is 0. Here,the TU size flag cannot be set to a value other than 0, since the sizeof the transformation unit cannot be less than 32×32.

As another example, (c) if the size of the current coding unit is 64×64and a maximum TU size flag is 1, then the TU size flag may be 0 or 1.Here, the TU size flag cannot be set to a value other than 0 or 1.

Thus, if it is defined that the maximum TU size flag is‘MaxTransformSizeIndex’, a minimum transformation unit size is‘MinTransformSize’, and a transformation unit size is ‘RootTuSize’ whenthe TU size flag is 0, then a current minimum transformation unit size‘CurrMinTuSize’ that can be determined in a current coding unit, may bedefined by Equation (1):

CurrMinTuSize=max(MinTransformSize,RootTuSize/(2̂MaxTransformSizeIndex))  (1)

Compared to the current minimum transformation unit size ‘CurrMinTuSize’that can be determined in the current coding unit, a transformation unitsize ‘RootTuSize’ when the TU size flag is 0 may denote a maximumtransformation unit size that can be selected in the system. In Equation(1), ‘RootTuSize/(2̂MaxTransformSizeIndex)’ denotes a transformation unitsize when the transformation unit size ‘RootTuSize’, when the TU sizeflag is 0, is split a number of times corresponding to the maximum TUsize flag, and ‘MinTransformSize’ denotes a minimum transformation size.Thus, a smaller value from among ‘RootTuSize/(2̂MaxTransformSizeIndex)’and ‘MinTransformSize’ may be the current minimum transformation unitsize ‘CurrMinTuSize’ that can be determined in the current coding unit.

According to an exemplary embodiment, the maximum transformation unitsize RootTuSize may vary according to the type of a prediction mode.

For example, if a current prediction mode is an inter mode, then‘RootTuSize’ may be determined by using Equation (2) below. In Equation(2), ‘MaxTransformSize’ denotes a maximum transformation unit size, and‘PUSize’ denotes a current prediction unit size.

RootTuSize=min(MaxTransformSize,PUSize)  (2)

That is, if the current prediction mode is the inter mode, thetransformation unit size ‘RootTuSize’, when the TU size flag is 0, maybe a smaller value from among the maximum transformation unit size andthe current prediction unit size.

If a prediction mode of a current partition unit is an intra mode,‘RootTuSize’ may be determined by using Equation (3) below. In Equation(3), ‘PartitionSize’ denotes the size of the current partition unit.

RootTuSize=min(MaxTransformSize,PartitionSize)  (3)

That is, if the current prediction mode is the intra mode, thetransformation unit size ‘RootTuSize’ when the TU size flag is 0 may bea smaller value from among the maximum transformation unit size and thesize of the current partition unit.

However, the current maximum transformation unit size ‘RootTuSize’ thatvaries according to the type of a prediction mode in a partition unit isjust an example and the exemplary embodiments are not limited thereto.

According to the video encoding method based on coding units having atree structure as described with reference to FIGS. 7 through 19, imagedata of a spatial region is encoded for each coding unit of a treestructure. According to the video decoding method based on coding unitshaving a tree structure, decoding is performed for each maximum codingunit to restore image data of a spatial region. Thus, a picture and avideo that is a picture sequence may be restored. The restored video maybe reproduced by a reproducing apparatus, stored in a storage medium, ortransmitted through a network.

Exemplary embodiments may be written as computer programs and may beimplemented in general-use digital computers that execute the programsusing a computer readable recording medium. Examples of the computerreadable recording medium include magnetic storage media (e.g., ROM,floppy disks, hard disks, etc.) and optical recording media (e.g.,CD-ROMs, or DVDs).

While exemplary embodiments been particularly shown and described withreference to the drawings, it will be understood by those of ordinaryskill in the art that various changes in form and details may be madetherein without departing from the spirit and scope of the inventiveconcept as defined by the following claims.

1. An apparatus for video decoding, the apparatus comprising: aprocessor configured to obtain, from a bitstream, collocated picturelist information, which indicates whether a collocated picture isdetermined according to a list L0 of a current block between the list L0and a list L1 of the current block, determine a collocated block in thecollocated picture determined using the collocated picture listinformation, when reference pictures, which are available for predictionof the current block, are to be output prior to a current pictureincluding the current block, determine a motion vector between a L0motion vector and a L1 motion vector of a collocated block according tothe list L0 or the list L1 of the current block, determine a motionvector predictor candidate according to the collocated block by usingthe determined motion vector, and obtain a motion vector predictor ofthe current block among predictor candidates including the motion vectorpredictor candidate according to the collocated block.